Light or medium wt. is ok, but the heavy duty has enough adhesive so that sewing on is not necessary ( think patching of holes). The light will not be a problem with a # 18 needle. The regular ( med.) you may need to slow down so the needle completes the stitch fully before you move it along. If there is an overlap (2 or 3 thicknesses) such as occurs with flower petals and a flower center, try to avoid stitching in that area ( you may get skipped stitches) . I take it you will be doing some kind of outlining and a few inside details rather than a panto. When doing an edge to edge, you may not be able to avoid those overlaps.